In the discussion to follow amplifier(s) is taken to mean a solid-state ultrashort optical pulse amplifier(s) of the regenerative or multipass type, or combinations of the two which use solid-state materials to amplify optical pulses with durations Full Width, Half Maximum, (FWHM), less than one nanosecond duration. These amplifiers have become an important tool for the study of the temporal behavior of phenomena in nature, for the study of nonlinear effects in physics, chemistry, electronics and biology, and for the generation of short pulses in the UV, VUV and X-ray regions. Their high peak power makes them ideal sources for producing ultrashort pulses at virtually any wavelength of interest through the use of processes like parametric amplification.
Heretofore, amplifier systems for generating ultrashort pulses are seeded by pulses that are generated in oscillators whose fundamental output wavelength falls within the gain bandwidth of the medium used to amplify them. A well known example is the commercially successful regenerative amplifier system that use Titanium-doped Sapphire as the gain medium. This system is seeded by a pulse generated in a mode-locked, external cavity oscillator containing a bulk Ti:Sapphire gain medium. These oscillators are in turn pumped by other lasers like an argon ion laser (or by a diode or diode array, if the gain medium were, for example, Li:SAF, Li:SGAF or Li:CAF). Although useful, this "laser-pumped laser" configurations have a number of undesirable characteristics. First, because regenerative amplifiers generally use some switching device like a Pockel Cell or Acousto-optic (A/O) switch that is driven by fast electronics to trap the seed pulse in the amplifier portion of the cavity, and because these devices have finite (several ns duration) rise times, the seed pulse train produced by this type of oscillator source must have pulse separation times that are sufficient to ensure that the Pockel Cell electronics can select only one pulse for injection from the train of pulses generated by the oscillator. Generally, most Ti:Sapphire oscillators have a cavity round trip time of about 10 ns. This is marginally sufficient to ensure that only one pulse is injected into the amplifier at a time, and places restrictions on the speed at which the electronics must operate. It would be desirable to increase the time separation between seed pulses to relax the requirements on the Pockel Cell or A/O switch, and also increase pre-pulse and post-pulse extinction ratios. This quickly becomes impractical with standard Ti:Sapphire oscillators because the greater temporal separation between seed pulses makes for a long cavity. Long external cavities possess greater sensitivity to misalignment which makes for less stable performance, and for a larger footprint.
An additional undesirable feature of known oscillator-amplifier combinations is that a Ti:Sapphire oscillator is itself pumped with an argon ion laser. Argon ion lasers are expensive to purchase, operate and maintain. And, like most laser-pumped lasers, precision alignment must be maintained between the pump beam and the active gain volume in the Ti:Sapphire rod. This is especially important when the seed laser oscillator is Kerr Lens or self-mode-locked, because these systems are extremely sensitive to even minor changes in the degree of overlap of the pump beam with the active gain volume in the Ti:Sapphire oscillator gain medium. This fact places additional restrictions on both the mechanical and thermal stability of the environment in which they can be operated. And because argon ion lasers are relatively large devices, their use in combination with a Ti:Sapphire oscillator makes for a bulky and expensive system that only a scientist could use. If this technology is to find widespread use in less esoteric environments than the research laboratory, it must be made less sensitive to the thermal and mechanical properties of the environment in which it is used. Additionally, the amount of space consumed must be kept to a minimum, the utility requirements readily available, and the power consumption within reasonable limits.
Lasers that use a doped-fiber as the gain medium can be made to have cavity lengths that are longer than equivalent external cavity designs--producing pulse trains with pulse separation times that are a factor of 2 to 100 times longer than those which are realistically achievable with external cavity configurations. A fiber gain medium can be coiled into a relatively small space so the effective footprint can be made quite small. A fiber laser gain medium can be diode-pumped, and because diodes are themselves small, efficient, have less stringent utility requirements, and do not consume much power, all the undesirable features of the argon ion pump laser can be eliminated by using a diode-pumped fiber seed laser. Moreover, a diode-pumped fiber laser based seed source can be made impervious to mechanical and thermal perturbations. All these characteristics give fiber lasers a robustness, and compactness that cannot be duplicated by traditional seed oscillator designs.
However, there are no diode-pumped fiber laser sources with emission wavelengths in the near IR where most of the broad gain bandwidth materials like Ti:Sapphire operate. Indeed, it is unlikely that an fiber-laser can be made to operate in this range, even if a suitable dopant material was available, because of large group velocity dispersion (GVD) in this wavelength region. Even within some existing fiber lasers like the Erbium-doped fiber oscillator, it has been found to be advantageous to incorporate GVD compensation mechanisms in the form of a negative GVD component to balance the positive GVD existing in the material at its operating wavelength, or use external normal or anomalous GVD mechanisms to compensate for chirp on the pulse.